1. Field of the Invention
This invention relates to the treatment of a hot aqueous brine solution containing various dissolved components, such as iron and silica to inhibit precipitation of scale, such as iron silicate scale, therefrom. More particularly, the invention relates to such a treatment wherein the scale is formed when the brine is produced and handled in a manner so that its temperature and pressure are reduced, e.g., a geothermal brine utilized for its heat content.
2. Description of the Prior Art
The solubility of most ions in solution decreases with a decrease in temperature of the solution. If dissolved ions are present near their saturation concentration in the solution, a slight reduction in the temperature of the systems can result in precipitation of a portion of these ions which often combine and deposit as a scale on any solid surface with which they come in contact, such as the vessel or conduit in which the solution is confined.
One example of such a solution is a hot water-containing fluid stream which is passed through a conduit in an industrial operation under conditions, such as lowering of the pressure, at which at least a portion of the hot water flashes to steam. If the hot water is a brine containing appreciable amounts of dissolved salts, this flashing is often accompanied by the formation of scale on the surfaces of the conduit contacted by the fluid stream. This deposition of scale tends to build up over a period of time and restrict further fluid flow through the conduit. This necessitates either operation at a reduced flow rate or an increase in the amount of power used to move the fluid through the conduit. In extreme cases the conduit can become completely plugged with scale, and the industrial operation must be shut down for maintenance. An example of such an industrial operation involves the generation of steam which can be used as a source of heat or to generate power. Various methods of generating steam utilize fossil-fuel steam generators, nuclear steam supply systems and geothermal generator units.
Large subterranean reservoirs of naturally occurring geothermal steam and/or hot aqueous liquid are found in many regions of the world. When readily accessible in advantageous locations, geothermal steam and water or brine have, for some time, been used for therapeutic purposes, industrial processes and/or direct heating. Although current interest in further developing geothermal resources for such purposes still exists, the principal effort has recently been directed more towards developing these resources, which are usually considered to be at least partially renewable, for production of electric power, the use of which is usually far less site-restricted than is the more direct use of the geothermal fluids for non-electric power purposes.
Techniques are known whereby hot geothermal fluids can be used to generate electric power. Geothermal steam can be used to drive combination steam turbine/electric generator apparatus. Pressurized geothermal water or brine, having a temperature above about 400.degree. F., can be flashed to a lower pressure to extract steam used for driving steam turbine/generators. In actual practice the problems encountered with handling and disposing of the large amounts of usually heavily contaminated and frequently highly saline geothermal liquids have often been quite formidable. Consequently, development of geothermal water/brine resources for production of commercial amounts of electricity has often been difficult and costly to achieve.
The most serious problems encountered with the use of hot aqueous liquids, such as geothermal brine, for producing electric power or in other uses usually result from severe scaling of the handling equipment, such as the confining vessels and conduits, used to contain the liquid. Because of their typically high temperatures and their long natural residence times in subterranean reservoirs, geothermal brines ordinarily leach large amounts of minerals from the reservoirs. These leached minerals typically include salts and oxides of heavy metals such as lead, zinc, iron, silver, cadmium and molybdenum. Other minerals such as calcium and sodium, generally in the form of chlorides, are also dissolved in the brine, as are naturally occurring gases, including carbon dioxide, hydrogen sulfide and methane. Large concentrations of silica, which may be in the form of silicic acid oligomers, are also commonly found dissolved in hot geothermal brines.
Various proposals have been made to decrease the scale formation in equipment used in producing and handling geothermal brine. In "Field Evaluation of Scale Control Methods: Acidification", by J. Z. Grens et al., Lawrence Livermore Laboratory, Geothermal Resources Council, Transactions, Vol. 1, May 1977, there is described an investigation of the scaling of turbine components wherein a geothermal brine having a pressure of 220 to 320 p.s.i.g. and a temperature of 200.degree. to 230.degree. C. (392.degree. to 446.degree. F.) was expanded through nozzles and impinged against static wearblades to a pressure of 1 atmosphere and a temperature of 102.degree. C. (215.degree. F.) In the nozzles the primary scale was heavy metal sulfides, such as lead sulfide, copper-iron sulfide, zinc sulfide and cuprous sulfide. The adherence of the primary scale of the metal substrate was promoted by thin basal layers of fine-grained, iron-rich amorphous silica. The scale formed on the wearblades was cuprous sulfide, native silver and lead sulfide in an iron-rich amorphous silica matrix. When the brine which originally had a pH of 5.4 to 5.8 was acidified with sufficient hydrochloric acid to reduce the pH of the expanded brine to values between 1.5 to 5.0, scaling was eliminated.
While the aforementioned treatments have met with some success in particular applications, the need exists for a further improved treating process to reduce scale deposition during the handling of hot aqueous brines, especially geothermal brines.
Accordingly, it is a principal object of this invention to provide a method for inhibiting the deposition of scale onto fluid handling equipment contacted by a hot water-containing fluid stream and the removal of such scale from such surfaces.
It is a further object of this invention to provide such a method for inhibiting the deposition of iron silicate scale from a geothermal brine.
It is a still further object of this invention to provide such a method wherein corrosion of the fluid handling equipment is minimized.
It is another object of this invention to treat a geothermal fluid, containing at least a portion of a geothermal brine, utilized for the generation of electric power so as to inhibit the deposition of iron silicate scale from the geothermal brine onto the fluid handling equipment.
Other objects, advantages and features of the invention will be apparent from the following description, drawing and appended claims.